1. Field of the Invention
The present invention relates to a drilling method, and to a drilling apparatus. Embodiments of the invention relate to a drilling method and apparatus where the effective circulating density (ECD) of drilling fluid (or drilling “mud”) in communication with a hydrocarbon-bearing formation is lower than would be the case in a conventional drilling operation. The invention also relates to an apparatus for reducing the buildup of drill cuttings or other solids in a borehole during a drilling operation; and to a method of performing underbalance drilling.
2. Description of the Related Art
When drilling boreholes for hydrocarbon extraction, it is common practice to circulate drilling fluid or “mud” downhole: drilling mud is pumped from the surface down a tubular drillstring to the drill bit, where the mud leaves the drillstring through jetting ports and returns to the surface via the annulus between the drillstring and the bore wall. The mud lubricates and cools the drill bit, supports the walls of the unlined bore, and carries dislodged rock particles or drill cuttings away from the drill bit and to the surface.
In recent years the deviation, depth and length of wells has increased, and during drilling the mud may be circulated through a bore several kilometers long. Pressure losses are induced in the mud as it flows through the drillstring, downhole motors, jetting ports, and then passes back to the surface through the annulus and around stabilisers, centralisers and the like. This adds to natural friction associated pressure loss as experienced by any flowing fluid.
Similarly, the pressure of the drilling mud at the drill bit and, most importantly, around the hydrocarbon-bearing formation, has tended to rise as well depth, length and deviation increase; during circulation, the pressure across the formation is the sum of the hydrostatic pressure relating to the height and density of the column of mud above the formation, and the additional pressure required to overcome the flow resistance experienced as the mud returns to surface through the annulus. Of course the mud pressure at the bit must also be sufficient to ensure that the mud flowrate through the annulus maintains the entrainment of the drill cuttings.
The mud pressure in a bore is often expressed in terms of the effective circulating density (ECD), which is represented as the ratio between the weight or pressure of mud and the weight of a corresponding column of water. Thus, the hydrostatic pressure or ECD at a drill bit may be around I.05SG, whereas during circulation the mud pressure, or ECD, may be as high as I.55SG.
It is now the case that the ECD of the drilling mud at the lower end of the bore where the bore intersects the hydrocarbon-bearing formations is placing a limit on the length and depth of bores which may be drilled and reservoirs accessed. In addition to mechanical considerations, such as top drive torque ratings and drill pipe strength, the increase in ECD at the formation may reach a level where the mud damages the formation, and in particular reduces the productivity of the formation. During drilling it is usually preferred that the mud pressure is higher than the fluid pressure in the hydrocarbon-bearing formation, such that the formation fluid does not flow into the bore. However, if the pressure differential exceeds a certain level, known as the fracture gradient, the mud will fracture the formation and begin to flow into the formation. In addition to loss of drilling fluid, fracturing also affects the production capabilities of a formation. Attempts have been made to minimise the effects of fracturing by injecting materials and compounds into bore to plug the pores in the formation. However, this increases drilling costs, is often of limited effectiveness, and tends to reduce the production capabilities of the formation.
High mud pressure also has a number of undesirable effects on drilling efficiency. In deviated bores the drillstring may lie in contact with the bore wall, and if the bore intersects a lower pressure formation the fluid pressure acting on the remainder of the string will tend to push the string against the bore wall, significantly increasing drag on the string; this may result in what is known as “differential sticking.”
It has also been suggested that high mud pressure at the bit reduces drilling efficiency, and this problem has been addressed in U.S. Pat. Nos. 4,049,066 (Richey) and 4,744,426 (Reed), the disclosures of which are incorporated herein by reference. Both documents disclose the provision of pump or fan arrangements in the annulus rearwardly of the bit, driven by mud passing through the drillstring, which reduces mud pressure at the bit. It is suggested that the disclosed arrangements improve jetting and the uplift of cuttings.
Another method of reducing the mud pressure at the bit is to improve drillstring design to minimise pressure losses in the annulus, and U.S. Pat. No. 4,823,891 (Hommani et al) discloses a stabiliser configuration which aims to minimise annulus pressure losses, and thus allow a desired mud flow to be achieved with lower initial mud pressure.
It is also known to aerate drilling mud, for example by addition of nitrogen gas, however the apparatus by necessary to implement this procedure is relatively expensive, cuttings suspension is poor, and the circulation of two phase fluids is problematic. The presence of low density gas in the mud may also make it difficult to “kill” a well in the event of an uncontrolled influx of hydrocarbon fluids into the wellbore.
It is among the objects of embodiments of the present invention to obviate or alleviate these and other difficulties associated with drilling operations.